Most toys/gadgets/walkmans/tape players/electronic object that needs something to move will have a motor of some kind inside it. These can be easily turned into pickups thusly...
In this demonstration I made a very basic instrument construction just to show how the pickup is used. I will talk about this in the last step. How you adapt your own version to your instrument depends entirely on your imagination!
Step 1: The Motor
Take off the plastic backing on the motor, the inside will look a bit like this, with the magnets around the edge and the coils around an axle in the middle.
Step 2: Small Holes
Drill a small hole inbetween the coils just about big enough to not destroy the magnets like what I did on my first attempt. Do the same on the back so that the string can travel all the way through...
its a good idea to glue the axle so that it doesn't spin.
Step 3: Small Solderings
Wire up the + and - terminals of the motor to a socket...
it doesn't matter which wire goes where.